Here's the html form with the GET variables that comprise the url.
<form method='post' action='http://www.pgdp.net/c/tools/project_manager/generate_post_files.php'>
<input type='hidden' name='projectid' value='projectID4b7deca7757f8'>
<input type='radio' name='round_id' value='[OCR]'>[OCR]
<input type='radio' name='round_id' value='P1' >P1
<input type='radio' name='round_id' value='P2' CHECKED>P2
<br>For each page, use:<br>
<input type='radio' name='which_text' value='EQ' CHECKED>the text (if any) saved in the selected round; or<br>
<input type='radio' name='which_text' value='LE'>the latest text saved in any round up to and including the selected round.<br>
(If every page has been saved in the selected round, then the two choices are equivalent.)<br>
<input type='hidden' name='include_proofers' value='0' /><input type='hidden' name='save_files' value='0' /><input type='submit' value='Download'>
</form>
All you need then is a list of the project codes.